1. Field of the Invention
This invention relates in general to microactuators for use in microdevices such as miniature valves, switches and sensors. The microactuators of the invention have application in fields such as air flow control for heating, ventilation and air conditioning, portable gas chromatographs, fluidic technology, end effectors for microrobotic manipulators, mass flow controllers in food and pharmaceutical processing and the like.
2. Description of the Related Art
Shape memory alloy ("SMA") materials such as Nitinol, an alloy of nearly equal atomic weights of Nickel and Titanium, are characterized in being easily deformed when cold (i.e. at a temperature below the material's phase change transition temperature), and which produce large stress forces, with shape recovery of several percent, when heated through the transition temperature in which the crystalline phase change is from martensite to austenite. One important application of SMA materials is for use in microactuators. U.S. Pat. No. 5,061,914 to Busch et al. issued Oct. 29, 1991 entitled Shape-Memory Alloy Micro-Actuator, assigned to TiNi Alloy Company of San Leandro, Calif., describes methods of producing microactuator elements which employ the vacuum deposition of thin film SMA onto substrates. U.S. Pat. No. 5,325,880 to Johnson et al. entitled Shape Memory Alloy Film Actuated Microvalve issued Jul. 5, 1994, and assigned to TiNi Alloy Company, discloses thin film SMA microvalves for use in controlling the flow of gas or liquid fluids. One advantage of SMA film in microactuators is its ability to produce large forces and displacements within small spaces at voltage levels which are compatible with electronics.
Macroscopic devices such as valves and sensors are typically assembled from individual components. In comparison, microactuators employing SMA film can be fabricated from monolithic silicon in methods which produce hundreds of devices on a single substrate in parallel processes. The cost per device is greatly reduced by such batch processing.
Micro-electro mechanical systems ("MEMS") in the future will require the integration of active as well as passive microdevices. Passive devices such as sensors have become a mature market, but active devices such as valves are not as widely used. The integration of passive and active mechanical devices, and eventually electronics, is a necessary step which is in nascent development. The progression is analogous to the development of solid-state electronics devices from resistors and capacitors to transistors, and then to integrated circuits.
During the past twenty-five years, research in the area of micro-fabricated sensors has revealed that the impact of micro-fabricated discrete sensors on biological and chemical applications remains limited. Practical applications in these fields require integrated analysis systems, not simply sensors. The attractiveness of the concept of a micro-fabricated integrated analysis system is that it will allow portable, rapid and complete measurements with the underlying analytical complexity hidden from the user. Potentially they will have great impact on many areas of application, such as analytical chemistry and measurements in environmental, biological and clinical chemistry. Typical analytical systems require sample handling steps such as mixing, dilution and calibration, thereby limiting the ability to miniaturize a complete system. For example, valves and pumps currently fulfill these important functions in handling liquid samples. In order to obtain the full benefit of micro-fabrication technology, these valves and pumps must also be micro-fabricated and integrated with the other system components.
Miniaturization of mechanical devices is motivated by portability, for example the portable gas chromatograph. Other considerations are cost and physical dimensions. In fluid systems for analysis of biological and chemical fluids, it is essential to reduce dead volumes to a minimum, otherwise analyses are not cost effective. A typical fluid control system requires a valve with a flow rate of microliter per minute at low pressure. Typically a miniature pneumatic valve is driven by a remote solenoid. This solution is unsatisfactory for several reasons. Besides being inelegant, it is impossible to construct very small pneumatic valves as, for example, arrays of valves on a single substrate. Further, the time behavior of the valves is influenced by the interconnections. A strong demand currently exists for an electrically powered actuator that is compact in size so that it may be placed in the immediate vicinity of a valve closure, yet forceful enough to provide adequate opening. SMA film technology addresses this need.
Among the other valve actuation mechanisms under development are those which employ electrostatic forces to open and close the valve. In such systems the high voltages that are required are incompatible with other electronic components. Another approach to microvalves includes the encapsulation of fluid phase-change devices, which have the disadvantage of slow actuation and the potential for fluid to escape. Another such microvalve approach is the use of piezoelectric actuators, which requires relatively more mass and volume to produce the required force and stroke.
Thin film actuators employing SMA can produce ten joules per cubic centimeter per cycle, which is at least an order of magnitude higher energy density than other types of actuator mechanisms. Using TiNi as the SMA material, the devices can be fabricated on silicon substrates which are then micro-machined using MEMS photolithography and selective etching techniques. Microribbons of TiNi have been produced that demonstrated repeated recovery of 3% strain under 30 ksi stress. Intrinsic materials properties are equal to or superior to those of bulk TiNi.